Method and apparatus for sputtering utilizing an apertured electrode and a pulsed substrate bias

ABSTRACT

COMBINING THE ADVANTAGES OF ION PLATING WITH THE VERSATILITY OF A RADIO FREQUENCY SPUTTERED SOURRCE. A PULSED HIGH VOLTAGE DIRECT CURRENT IS PASSED TO THE ARTICLE BEING PLATED DURING RADIO FREQUENCY SPUTTERING.

May 8, 1973 JFS METHOD AND APPARAT .PRZYBYSZEWSKI ET AL us FORSPUTTERING UTILIZING AN APERTURED v MATCHING NETWORK RADIO FREQUENCYPOWER SOURCE as Er HIGH VOLTAGE 0.0. (0-5 Kv.) R

TIMED SWiTCH I mvmrons JOHN S. PRZYBYSZEWSKI ATTORNEYS United StatesPatent O METHOD AND APPARATUS FOR SPUTTERING UTILIZING AN APERTUREDELECTRODE AND A PULSED SUBSTRATE BIAS John S. Przybyszewski, NorthOlmsted, and Richard K. Shaltens, Lakewood, Ohio, assignors to theUnited States of America as represented by the Administrator of theNational Aeronautics and Space Administration Filed Jan. 14, 1971, Ser.No. 106,424 Int. Cl. C23c 15/00 US. Cl. 204-492 7 Claims ABSTRACT OF THEDISCLOSURE Combining the advantages of ion plating with the versatilityof a radio frequency sputtered source. A pulsed high voltage directcurrent is passed to the article being plated during radio frequencysputtering.

ORIGIN OF THE INVENTION The invention described herein was made byemployees of the United States Government and may be manufactured andused by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention is concerned with platingadherent films on objects having complex geometries. The invention isparticularly directed to ion plating alloy films on such objects using aradio frequency sputtered source. The ion plating process is modifiedbecause this source supplies film material at a much slower rate thanthe usual thermal evaporation source.

In the past several methods were used to deposit various types of filmson simple as well as geometrically complex objects. While each processis satisfactory for certain applications, problems have been encounteredwith all of these methods.

Conventional vapor deposition is conducted in a vacuum of 10* to 10-torr. The use of conventional vapor deposition has been generallyrestricted to the elemental metals, although some metal alloy systems aswell as certain semiconductors and nonconductors have been vapordeposited. The high vacuum used in vapor deposition reduces theconcentration of gas molecules which increases the mean free path. Verylittle scattering of the film material results, and the process islimited to line-of-sight deposition. The coating of complex geometriesby vapor deposition is conditioned on the rotation of the object to becoated. Adherence of a vapor deposited film is poor because of the lowenergy 'of the impinging film material.

The film adhesion is improved when either direct current or radiofrequency sputtering is used. Direct current sputtering has beensuccessful for depositing elemental metals, semiconductors, and metalalloy systems. This type of sputtering is not useful for depositingnonconductors. Radio frequency sputtering has been used for depositingelemental metals, metal alloy systems, semiconductors, andnonconductors. This type of sputtering is not limited by the nature ofthe film material. It can be used to sputter almost any material frominsulators through semiconductors to metals.

Radio frequency and direct current sputtering are generally done in anatmosphere having a pressure in the range of about to 20 microns.Because of this relatively high pressure the sputtered material isscattered. The mean free path is short, and the material is diffusedrapidly as it leaves the source. Eventhough this scattering effectcauses film formation on surfaces not directly facing the sourcematerial, both RF and DC sputtering are considered to be line-of-sightdeposition processes. The low energy of the impinging film materialadversely affects film adherence.

Ion plating is performed at about the same pressure as RF and DCsputtering; A high voltage is applied to the object to be coated. Thisresults in a uniform coating on all sides without rotating or movingeither the object or the source of film material. While the coating hasexcellent adhesion, problems have been encountered because the processutilizes a thermal evaporation source. This limits the film materials tothe elemental metals and those com pounds which do not dissociate beforethey evaporate.

SUMMARY OF THE INVENTION These problems have been solved by the presentinvention which utilizes radio frequency sputtering with a pulsed highvoltage direct current. The process is not limited to a line-ofsightdeposition, and complex geometries can be plated without rotation. Theprocess is useful for plating adherent films of elemental metals, metalalloy systems, semiconductors, and nonconductors.

OBJECTS OF THE INVENTION It is, therefore, an object of the presentinvention to plate an adherent alloy film on an object having ageometrically complex configuration.

Another object of the invention is to provide an improved plating methodwhich combines the advantages of ion plating with the versatility of aradio frequency sputtered source.

A further object of the invention is to provide an improved method forplating alloy films on complex geomctries without rotation during theplating process.

These and other objects of the invention will be apparent from thespecification which follows and from the drawing wherein like numeralsare used throughout to identify like parts.

DESCRIPTION OF THE DRAWING The figure is a schematic diagram of a systemconstructed in accordance with the invention for plating adherentalloyed films on geometrically complex objects.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingthere is shown an object 10 which is to be coated in accordance with thepresent invention. The object 10 may be any electrically conductivearticle having either a simple or geometrically complex configuration.By way of example the invention has been utilized to coat bearings witha solid lubricant. The object 10 is mounted in a chamber 12 that isconnected at 14 to a suitable vacuum pumping system.

A target 16 of the material to be sputtered is likewise located in thechamber 12. The target 16 is connected to a radio frequency power source18 through a matching network 20. This RF sputtered source 16 isutilized instead of a thermal evaporation source normally used in ionplating. As stated earlier, certain modifications are required becausethe RF sputtered source 16 supplies film material at a much slower ratethan a thermal evaporation source.

A suitable gas is supplied to the chamber 12 at an inlet 22. Argon ispreferably used. The object 10 to be plated is connected to a highvoltage, direct current source 24. The source 24 preferably has a rangeof O to 5 kilovolts.

In this manner the ion plating is carried out in a low pressure ionizedgaseous atmosphere with the object 10 to be plated forming a cathodethat is maintained at a high negative potential from the source 24.Consequently, the object 10 to be plated is continuously bombarded orsputtered by ions before, during and after film material enters theionized gas. If the evaporation rate of the film material is too slowthe film on the object to be plated will be sputtered away as fast as itdevelops, and no film will result. Because the RF sputtered source 16 isinherently slow, no film would develop under normal ion platingconditions.

According to the present invention the ion plating process has beenmodified to reduce the rate of sputtering off of the newly formed filmby providing a timed switch 26 between the source 24 and the object 10.The reduction in the sputtering off rate is accomplished by pulsing thenegative high voltage DC from the source 24 to the object 10 by means ofthe timed switch 26.

A third electrode 28 is positioned in the chamber 12 to establish acommon electrode between the RF power source 18 and the DC power source24. The electrode 28 is connected to the high voltage DC power source 24through an RF choke 30. The electrode 28 forms an anode with respect tothe cathode 10. The radio frequency power source 18 is connected to theelectrode 28 through a capacitor 32. A bypass capacitor 34 is likewiseprovided.

This third electrode 28 is preferably in the form of a perforated plateor screen that is located between the sputtered source 16 and the object10. The screen has an aperture in the center to enable sputteredmaterial to pass to the object to be plated.

In operation, the object 10 is mounted in the chamber 12 together withthe target 16 of material to be sputtered. The chamber 12 is partiallyevacuated and a gaseous atmosphere from about 10-20 microns pressure isestablished. A high voltage DC negative potential of 2 to kilovolts withrespect to the screen 28 is continuously applied to the object 10. Thisestablishes a glow discharge within the vacuum chamber 12 to sputterclean the object 10.

After a predetermined period of sputter cleaning, the high voltage DCsource 24 is deenergized and the gaseous pressure is lowered to aboutmicrons. The RF power source 18 to the film material 16 is energized,and sputtering of the film material begins. At this point the highvoltage DC source 24 is switched to a timed on-oif mode by the switch26. In this manner the high voltage DC is reenergized and reapplied tothe object 10.

The pulsed high voltage direct current RF sputtering process produces anintense electric field which completely surrounds the object 10. Thiscan be seen as a dark space around this object. Any ionized materialentering this region gains kinetic energy from the field and impacts onthe surface of the object 10 with great force. This contributes toimproved adhesion. The dark space, in effect, represents essentially asource of film material which takes on the general outline of the object10.

The high voltage DC negative potential on the object 10 is maintainedthroughout the plating. The process is continued until the desired filmis obtained. The reduced sputtering rate of the film on the object 10results in the formation of a visible film having excellent adherenceand covering the entire object.

EXAMPLE Pulsed high voltage direct current radio frequency sputteringwas used to plate antifriction bearing components with a solid lubricantLfill'Il of molybdenum disulfide. The plating conditions are as follows:

Total coating time each component: 3 hours Radio Frequency Input Powerto ,Source Material: 700

watts at 7 megahertz Maximum pulse amplitude to specimen: 2000 V-DC,

negative Pulse form: seconds on; 5 minutes off; 5% duty cycle Specimento screen distance: 2.5 inches Specimen to source distance:approximately 6 inches Chamber pressure: 5 microns; argon The componentswere assembled and the bearing was tested. The hearing was satisfactoryfor its intended use.

While one embodiment of the invention has been shown and described itwill be apperciated that various modifications to the invention may bemade without departing from the spirit of the invention or the scope ofthe subjoined claims.

What is claimed is:

1. In a sputtering apparatus, including a vacuum chamber, means foradmitting a gas into said chamber, target holding means for supportingthe material to be sputtered, substrate holding means for supporting asubstrate to be coated, means for applying RF potential to said targetto sputter said material, and means for applying a high voltage directcurrent to said substrate;

the improvement wherein an apertured electrode is disposed between saidsubstrate holding means and said target holding means, said electrodebeing connected to said means for applying RF potential and said meansfor applying a high voltage direct current whereby a high voltage directcurrent negative potential with respect to said electrode is applied tosaid substrate holding means and an RF potential is applied to saidtarget holding means to sputter said material, and

timed switch means connected to said means for applying a high voltagedirect current, said timed switch means enabling said high voltagedirect current negative potential to be pulsed from of about 15 secondson and about 5 minutes off.

2. Apparatus as claimed in claim 1 wherein said apertured electrode is ascreen.

3. Apparatus as claimed in claim 2 wherein the screen has a centrallydisposed aperture therein.

4. In an RF sputtering process wherein an RF potential is applied to atarget to sputter material from the target onto a substrate, theimprovement comprising disposing an apertured electrode between thetarget and the substrate, connecting said electrode to a source of RFpotential and to a source of high voltage direct current, and applyingto said substrate a high voltage direct current negative potential pulsefrom of about 15 seconds on and about 5 minutes off.

5. A method as claimed in claim 4 wherein said pulsed voltage is apotential of about 2 to 5 kilovolts.

6. A method as claimed in claim 4 including disposing said target about2.5 inches from said apertured electrode and said substrate about 6inches from said apertured electrode.

7. A method as claimed in claim 6 wherein argon is utilized at apressure of about 10 microns.

References Cited UNITED STATES PATENTS 3,361,659 1/1968 Bertelsen204-298 3,526,584 9/1970 Shaw 204-298 3,528,906 9/1970 Cash et a1204-295 3,461,054 8/1969 Uratny 204-192 3,479,269 11/1969 Byrnes et al.204-192 3,530,055 9/ 1970 Maissel et al 204-l92 3,589,994 6/1971Schwartz et al. 204-192 JOHN H. MACK, Primary Examiner S. S. KANTER,Assistant Examiner US. Cl. X.R. 204298

